1. Field of the Invention
The present invention relates to a method of producing a powdery coating material comprising a curable acrylic resin.
2. Description of the Related Art
Powdery coating materials do not generate volatile organic substances during the baking, do not cause environmental problems such as air pollution, and have, hence, been used in a wide field of applications to substitute for the solution coating materials.
As such powdery coating materials, there has been known an acrylic powdery coating material containing, for example, a curable acrylic resin having a glycidyl group and a curing agent as represented by an aliphatic dibasic acid.
The powdery coating material is generally prepared by dry-mixing a curable resin, a curing agent, additives for the coating material and pigment, kneading and dispersing them by using a melt kneader, followed by the pulverization and classification. In heating and melt-kneading the curable resin and the curing agent according to this method, however, it is necessary to suppress the crosslinking reaction between the curable resin and the curing agent. Therefore, limitation is imposed such as effecting the kneading at a temperature lower than the melting point of the curing agent, making it difficult to homogeneously disperse the resin and the curing agent. Further, the film formed by using the thus produced powdery coating material has problems concerning the appearance and, particularly, lacking smoothness. Besides, the pigment is poorly dispersed making it difficult to obtain a film that exhibits vividness comparable to that of solution coating materials.
In order to improve such problems, there have been proposed methods of mixing a curable resin and a curing agent in a wet manner, i.e., in a solvent (Japanese Unexamined Patent Publications (Kokai) Nos. 25531/1979, 53729/1998, 302567/1999, 103866/2000).
Even with these methods, however, the curable resin and the curing agent are not necessarily dispersed sufficiently homogeneously, still leaving many problems such as the necessity of vessels for dissolving starting coating materials, mixing devices for mixing the solutions thereof, and a large apparatus for vapor-removing/recovering large amounts of solvent.
U.S. Pat. No. 6,309,572 discloses a method of producing a powder coating material in which a curable acrylic resin is prepared in methanol and the obtained resin solution is wet-mixed with a curing agent. Methanol has a low boiling point and is vapor-removed easily. However, when methanol is used in the process for producing the acrylic resin and in the process for the powder coating material, methanol contacts with a glycidyl group and/or a methyl glycidyl group contained in the acrylic resin for a long time during being heated. In the result, the ring of the glycidyl group or the methyl glycidyl group is opened by methanol, therefore, the anti-blocking property of the obtained powder coating material and the mechanical property of the film are impaired.